This invention is concerned with detecting, locating, identifying, and tracking submerged objects, particularly submarines, through the transmission and reception of sonar pulses called "pings". Underwater acoustic surveillance systems and methods that now carry out these tasks are usually divided into two categories designated as "active" and "passive", although some mixed systems are also employed. Active underwater surveillance involves transmitting sonar signals, pings, then intercepting and analyzing the reflected or scattered echoes from the objects these signals may encounter. The analyzed signals may yield information determining the presence, location, identity, and track of these underwater objects.
Active underwater sonar surveillance systems are usually monostatic, i.e., the same platform is used for transmitting and intercepting these signals. A disadvantage to using an active system is that a targeted object may be able to trace the signal back to the transmitter platform. Should the target be hostile, such as an enemy submarine, it may be able to use this information to interdict the transmitter platform or to take evasive action. Submarines have also adopted, as stealth countermeasures, hull designs and construction materials that absorb or diffuse sonar signals making these reflected echoes difficult to detect and analyze.
In passive acoustic underwater surveillance, listening platforms employ hydrophones to monitor the underwater sounds present in the ocean environment. Recognition of characteristic sounds, called "acoustic signatures", of various vessels lead to their detection and identification. Submarines, however, have become quieter and are developing increasingly more effective stealth techniques enabling them to more successfully evade detection by passive systems. Also, a submerged object which is emitting no sound, such as a stationary submarine, would be undetectable to a purely passive system.